1. Field of the Invention
This invention relates to a screen printing method for printing solder paste, etc., onto a work through a screen mask, and a printing apparatus. More particularly, the invention relates to a screen printing method, and an apparatus for the method, that will be suitable for providing screen printing on a high density multi-layered substrate, and a printing apparatus.
2. Description of the Related Art
To connect a printed substrate and a semiconductor chip, a connection system called a “flip-chip (FC) system”, that uses solder bumps (minute solder protrusions), has generally been used. This system can connect, at one time, thousands or dozens of thousands of connection points by arranging semiconductor bumps in a plane. With the progress of IT technologies, semiconductor chips have higher functions and the connection of such semiconductor chips requires a more minute size, a higher density and a greater number of connection points.
To form the solder bumps, a prior art method uses an apparatus executing two steps at a work alignment position and a solder printing position, sets a work (substrate) to the work alignment position, moves the work to the solder printing position, then superposes a metal screen mask (hereinafter called “screen”) on the work by using a screen elevation mechanism and prints the solder paste to the work for printing through a squeegee, or the like (refer to Japanese Unexamined Patent Publication No. 6-155706, for example).
FIGS. 4 and 5 of the accompanying drawings show this screen printing apparatus according to the prior art. The screen printing machine 1 includes a work alignment mechanism 2 for positioning a work (substrate) W, a work movement mechanism 3 for causing the work W to linearly reciprocate between a work alignment position and a solder printing position, a screen movement mechanism 4 for causing a screen S to linearly reciprocate between the work alignment position and the solder printing machine, a screen elevation mechanism 5 for moving the screen S up and down, a solder bump printing mechanism 6, an image processing camera 7, a camera elevation mechanism 8, and so forth. In this printing apparatus 1, the work W is put on a table 21 of the work alignment mechanism 2 and is positioned by the work alignment mechanism 2 at the work alignment position on the basis of the image taken by the image processing camera 7. Next, the work W is moved to the solder printing position by the work movement mechanism 3 and the screen S is superposed on the work at this solder printing position by the screen movement mechanism 4 and the screen elevation mechanism 5. A squeegee 61 of the solder bump printing mechanism 6 is moved and the solder paste P is applied to the work W through the screen S to thereby print the solder bumps B onto the work W.
Generally, a decisive factor in the printing quality of the solder bumps is the precision of superposition of the printing pattern of the screen S onto the screen pattern of the work W. Because an opaque metal screen S is used, it is not possible to superpose the work W and the screen S one upon another and then to position them. Therefore, a system that individually aligns a positioning marker M of the metal screen S and a positioning marker of the work W by using an image processor and mechanically superposes the screen S and the work W one upon another on the basis of the image processing data has been employed.
When the image processing camera 7 is stationary as shown in FIGS. 4 and 5, therefore, the system becomes the printing system that moves the screen S to the work alignment position by using the screen movement mechanism, aligns the screen S by using the image processing camera 7 and then returns the screen to the solder printing position to conduct printing. Therefore, the number of moving mechanisms, such as those for the movement and elevation of the screen S and for the elevation of the image processing camera 7, is large and the set position of the screen S after positioning contains mechanical errors, from the individual movement mechanisms, during alignment.
In addition, because the positioning marker (non-penetrating due to half etching) disposed in the screen S and a printing port (penetrating) for practically soldering the solder have different processing conditions, processing is not simultaneous processing and processing errors occur. This error is further increased by a camera movement system.
Therefore, it has become difficult, in recent years, for such a solder printing mechanism of the prior art and including complicated mechanisms, to satisfy the requirements for further miniaturization, higher density and a greater number of connection points owing to the structural errors.
In a high density multi-layered substrate of the next generation, in particular, the number of bumps per work is thousands of bumps and is more than ten times the number of the existing bumps that is in the order of hundreds. Therefore, the bump density also reaches a high density of bump diameter φ100×pitch 150 μm. The solder paste printing method of the prior art alignment system cannot easily cope with such a requirement.
Further, according to the PALAP (Patterned prepreg Lay-up Process) substrate filed by the present applicant, the substrate material has high wettability with solder unlike the conventional glass-epoxy substrates. Unless the solder is correctly transferred to the bump formation positions, the solder flows during re-flow (because the solder is molten in a non-oxidizing atmosphere to form balls through surface tension) and connects adjacent bumps to cause a bridge defect. Therefore, a printing technology not inviting oozing of the solder, resulting from floating of the screen, is necessary.